This invention relates to a novel method of preparing polymer based powder coating compositions and, more particularly, to a method of preparing such polymer based powder coating compositions featuring the use of ionizing radiation.
In recent years, organic polymer based powder coating compositions have gained in interest to those in the coatings industry because of environmental pollution concerns and the increasing cost of petroleum based organic solvents.
Various prior methods of preparing such polymer based powder coating compositions have been employed. In one early method, the polymer and other ingredients necessary to formulate this coating composition preferably in solid form, were mixed together in a suitable mixing apparatus, the mixture was then extruded and the extruded material then subjected to a grinding process to obtain the powder coating material. While this method of preparing polymer based powder coating compositions insured intimate mixing of the various ingredients, it had several disadvantages. Thus, the method was a complex one which involved precrushing, premixing, extruding, cooling, grinding, sieving and dust collection. Furthermore, complete displacement of the processed material in the extruder was necessary to assure a uniform and homogeneous product while color matching was difficult to achieve from batch to batch.
More recent methods of preparing such polymer based powder coating compositions have employed a solution or non-aqueous dispersion process. However, both of these processes have a number of serious limitations.
The solution method, as exemplified in U.S. Pat. Nos. 3,730,930; 3,770,848 and 3,787,521 involves the formation of the polymer in solution and the subsequent drying (e.g., evaporation or spray drying) of the solution to obtain the powder. This solution method has several disadvantages. Thus, for example, small amounts of entrained solvent remaining after the drying process may cause tackiness and particle agglomeration. Further, the powder particles produced by this method usually have low bulk density characteristics which may lead to poor deposition efficiency. Finally, since highly volatile solvents are normally used in the spray drying of solution polymers, solvent reclamation is difficult and expensive while recovery of very fine particles is often difficult.
The non-aqueous dispersion method, as exemplified in U.S. Pat. Nos. 3,166,524; 3,218,287; 3,382,297 and 3,691,123 involves the formation of the polymer in an organic dispersing liquid in which the monomers are soluble but in which the resultant polymer is insoluble thereby forming a polymer dispersion and the subsequent drying of the dispersion to obtain the powder. The non-aqueous dispersion method while having significant advantages compared to the solution method also possesses several significant disadvantages. Thus, for example, in the formation of the polymer dispersion, complex and expensive dispersion stabilizers are normally required. Moreover, while not as great a problem as encountered in the solution method, minor amounts of entrained solvent may remain in the particles and solvent reclamation problems still exist.
The method of the present invention substantially obviates most, if not all of these disadvantages. Thus, in the method of this invention, the use of organic solvents, organic dispersing liquids, and complex dispersion stabilizers for polymerization is completely eliminated. The method of the invention is characterized by its convenience and simplicity of operation.